Method of treating oil wells yielding gas



'5 It more particularly concerns an Patented Feb. 14, 1939 UNITED STATES PATENT OFFICE METHOD OF TREATING OIL WELLS YIELD- ING GAS Orie N. Maness, Midland, Tex., assignor to The Dow Chemical Company, Midland, Mich., a

corporation of Michigan No Drawing. Application August-13, 1936, Serial No. 95,839

3 Claims.

The invention relates to methods of increasing the production of oil wells by treatment with a reagent such as an acid capable of facilitating the flow of oil from the producing formation. improved method of treating oil wells wherein the .treating agent is forced into the formation below a predetermined level under the pressure of a column of oil in the well bore. A well known method of the foregoing character is disclosed in United States Patent No.

1,891,667. In that method the well cavity, tube, and easing are filled with oil which is immiscible with the aqueous acid and of lower specific gravity, and then the acid is introduced through the tubing into the cavity while allowing oil displaced from the cavity and tubing to be discharged from the casing head until the acid rises up to the desired level in the bore, which is usually about the top of the producing horizon, but below the casing seat. The casing is then closed and the hydrostatic pressure created by the acid standing in the tube, augmented, if necessary, by pumping in additional oil or acid into the tube, causes the acid to enter the producing formation below the predetermined level maintained by the oil in the casing. In addition to preventing the acid from rising up in the casing beyond a predetermined level, the oil column in the casing g0 and tubing also has the function of exerting pressure on the acid in the well cavity.

I have found that in oil wells yielding gas under a sufiicient pressure to aerate the oil column in the well such method cannot be used satisfactorily to ensure the delivery of the reagent into the producing horizon below a predetermined level. The aerated oil column is elastic and readily compressible so that when it is attempted to introduce acid into the well and con- 40 trol the level at which it enters the formation by discharging oil from the casing until the acid is thought to have reached the desired level and then closing the casing, the acid continues to rise compressing the oil in the casing. Positive assurance of delivering the acid into the formation below a predetermined level thus cannot be had when the oil in the casing is aerated. The acid may, and often does, rise above the oil-producing horizon and enter the formation at a level which does not produce oil or only a minor portion thereof' and hence may be ineffective to increase the oil production. If the acid enters a gasproducing stratum, soluble in acid, overlying an oil-producing stratum, the output of gas will be increased at the expense of the oil output.

I have found it is not possible in such cases to prevent aeration of the oil or obtain a dead oil column in the casing by circulating oil down the tubing and up the casing or vice versa, as in conventional, practice with ordinary wells, with any reasonable volume of oil, because the oil is instead forced into the formation when sufficient pressure is applied to prevent the gas entering the well. Attempts to pump oil into 'the well against the gas pressure result not only in excessive pressures being developed at the well head in some cases but also in large losses of oil which enters the formation at the lowest levels owing to the relatively large pressure head there created. Gas will enter the well also, even after it seemingly has been filled with oil in conventional manner, because of the difference inthe density of gas and oil. It is, therefore, practically impossible to control hydrostatically a gas-producing oil well by the conventional method when the gas pressure is sufficient to aerate the oil standing or circulated in the well bore.

Among the objects of my invention, then, is to provide a method of hydrostatically controlling an oil well yielding gas, whereby a treating agent can be introduced into the well under the pressure of an oil column positively ensuring delivery of the reagent into the producing horizon at the proper level. Other objects and advantages will appear as the description proceeds.

In carrying out the invention I introduce oil into the well and together therewith a quantity of a finely divided oil-insoluble but acid-soluble material to obtain hydrostatic control of the well before introducing the acid reagent. As the oil is introduced into the well, which can be done either through the casing or the tubing, gas is vented from the well in order to discharge the gas accumulated in the well bore and to facilitate the introduction of the oil. It is preferable to introduce oil alone at first into the tubing, while carefully venting gas from the casing, because it is usually possible to kill the tubing pressure in that way, since the gas issuing from the formation generally will rise up the casing instead of the tubing once oil is introduced down the tubing. While introducing oil down the tubing there is introduced therewith a quantity of a material which will form an oil-insoluble but acid-soluble sealing deposit on or in the walls of the lower portions of the bore hole through which fluids enter the well or can be forced. therefrom into the formation. For this purpose I use a finely divided water-insoluble oxide, hydroxide, 'sulphide, or carbonate'such as, for example, powup, either in oil or in water, and introduced into the well with the oil. If desired, a suspension of the material can be made by mixing two solutions which react together to form a precipitate of finely divided acid-soluble material. For example, a solution of sodium carbonate or hydroxide and a solution of calcium chloride may be mixed together and introduced into the well forming a precipitate of calcium carbonate or hydroxide, respectively. A suspension contain-' ing-from 0.5 to 2 lbs. of finely divided material per gallon of vehicle (i. e. oil or water) is a suitable concentration but other proportions may be used.

The oil introduced into the well enters the po rous portions of the formation and carries the finely divided acid-soluble material with it, forming a deposit in the formation preventing further passage of oil thereinto. Such deposit will be formed mainly in or upon those portions of the formation not producing gas when gas is being vented from the well while introducing the oil and sealing material. The deposit also prevents gas leakage from the formation into the bore provided the deposit is formed on a gas-producing portion;

The introduction of oil and finely divided acidsoluble material with the aid of oil down the tubing is thus not always effective to seal off a gasproducing portion of the formation when this is much higher in the bore than the lower end of the tubing or when the gas formation is at a considerable height above the lower portion of the bore. Under such circumstances I follow the introduction of oil. and sealing material down the tubing with more oil and sealing material down the casing while carefully venting gas from the casing, if necessary, until the gas pressure at the casing head is substantially reduced or overcome. When the gas pressure is overcome it indicates that the sealing material has been deposited upon the face of the gas and oil-producing portions of the formation preventing the issuance of gas therefrom into the well bore and allowing a solid column or body of dead oil to be formed in the well bore.

As a result of the foregoing steps the well is filled with oil and put under hydrostatic control. Loss of oil into'the formation and issuance of gas therefrom is prevented by the finely divided material which is deposited over the face of the formation and held in place by the oil in the well bore.,

The acid-treating agent is then introduced into the well through the tubing and, being heavier than the oil, settles to the bottom of the well. The level to which the acid rises in the bore outside the tubing is controllable by regulating the pressure head of the oil in the casing, as by discharging or introducing oil at the casing head. Ordinarily it is not desirable to discharge more oil than the volume of the well tube, because when the acid has reached the bottom of the bore, there is then a risk, in releasing the oil pressure in the casing, of gas issuing from the upper portion of the producing formation and aerating the oil column which would reduce the hydrostatic head therein and permit the acid to rise in the casing instead of being held down at the lower oil-producing portion of the formation. The risk of aerating the oil column can be prevented by maintaining oil pressure at the casing head equal to, or greater than, the difference between the hydrostatic head of the acid in the tubing and that of the oil in the casing less the head of the acid outside the tubing at the level to which it is desired that the acid rise in the bore. This can be accomplished preferably by introducing through the casing a quantity of oil. Such oil can be introduced simultaneously with the introduction of the acid reagent or intermittently therewith after the initial portion has reached the bottom of the bore. Proper positioning of the acid level can thus be obtained by regulating the casing head pressure once hydrostatic control of the well is established and maintained by the oil when the acid-treating agent in the tubing has reached the bottom of the bore.

Any acid capable of attacking and dissolving the rock may be employed, such as hydrochloric, nitric, or mixtures of such acids. I prefer to use a 5 to 20 per cent solution of hydrochloric acid to which is added an inhibitor as described in United States Patent No. 1,877,504, when the formation comprises calcareous matter.

When the acid comes into contact with the well bore it begins to act upon it and the deposit of finely divided material, which, being acid-soluble, is readily removed by the acid. The acid can then enter the formation well below the gasproduci'ng portion when suflicient head is available to overcome the formation pressure. The

acid solution may be forced into the formation, if

necessary, by adding more acid through the well tube or by introducing oil down the tubing. When the acid enters the formation a reduction of pressure may occur at the casing head lowering the hydrostatic pressure on the upper portions of the formation sufficiently to permit the gas pressure to blow off the sealing deposit at this point and thus allow gas to enter the well. In the event the pressure were reduced the oil column in the casing would become aerated and this would reduce the hydrostatic head created by it upon the acid allowing the acid to rise in the casing above the producing horizon to be treated. In order to guard against such reduction of pressure, oil may be forced down the easing while acid is being introduced down the tubing. Such introduction of oil down the casing while acid is being introduced down the tubing and forced into the formation ensures protecting the sealing deposit on the upper portions of the' formation and the prevention of loss of pressure holding the acid down opposite the producing horizon into which the acid is to be delivered.

After the desired amount of acid has been introduced into the formation, which may be from less than 500 gallons to several thousand gallons, the well may be shut in permitting the solvent action of the acid to take place before releasing the pressure and allowing the well to produce.

I Among the advantages of the method are that the applied pressures required to force the acid reagentinto the well are greatly reduced; loss of oil into the formation is minimized so that a comparatively small volume suffices to put the well under hydrostatic control; the delivery of the acid reagent into the earth formation at a predetermined level is positively assured; detrimental or ineffective action of the reagent on the formation is practically prevented; and the ratio of the production rate of oil to that of gas is substantially increased, thereby conserving the formation pressure.

Other modes of applying the principle of the invention may be employed instead of thoseexany of the following claims or the equivalent of such stated steps be employed.

I therefore particularly point out and distinctly claim as my invention:-

1. In acid treating an oil well producing gas by the method employing a hydrostatic oil column in the well bore to exert pressure on the acid charge, wherein the porous character of the formation normally prevents the establishment of such oil column due to drainage into the formation, the steps which consist in introducing oil and together therewith a finely divided oiland water-insoluble but acid-soluble solid material into the well bore to form a column of sufficient height to overcome the gas pressure in the well, whereby such finely divided material is carried oil.

by the oil and deposited in the pores of the formation to seal them and prevent drainage of oil thereinto, then introducing a charge of an acid reagent into the well while permitting the oil therein thereby displaced to escape from the well head until the acid settles to the bottom of the well and reaches the desired level in the bore but acid-soluble solid material as a suspension in water.

3. In the method according to claim 1 employing the finely divided oiland water-insoluble but acid-soluble solid material as a suspension in ORIE N. MANESS. 

